1. Field of the Invention
The Present invention relates to an image forming apparatus such as a copier, a printer, a facsimile and the like and more particularly, it relates to an image forming apparatus capable of conveying a sheet material in normal and reverse directions.
2. Related Background Art
In conventional image forming apparatuses of ink jet type, an image is formed by shifting and scanning a recording head for discharging ink in a direction transverse to a conveying direction of a sheet material fed from a sheet feeding portion in an image forming portion and by discharging the ink onto the sheet material, and the imaged sheet material is discharged into a discharge portion. In the image forming apparatuses of this type, for example, when the image is formed on a single sheet material having A4 size, since image formation is completed by reciprocating the recording head by several times, a recording speed is about several sheets per one minute.
However, in recent years, it has been requested that the image forming speed be further increased, and, in order to satisfy such request, there has been proposed an image forming apparatus having an ink jet recording head of so-called full line type in which ink discharge ports are arranged across a whole width of a recording medium in a direction transverse to the conveying direction of the sheet material thereby to increase the recording speed to ten-odd sheets per one minute.
By the way, in the image forming apparatus of ink jet type, when a sheet material having a front surface on which the image was formed is conveyed at a downstream side of a recording head disposed in an image forming portion in a conveying direction, the sheet material may be contacted with a sheet material conveying member such as a discharge roller before ink as a recorded image on the sheet material is dried and fixed, with the result that there arises inconvenience that the image formed on the sheet material is distorted.
Further, there may arise inconvenience that the succeeding sheet material is contaminated by transferring the ink adhered to the surface itself of the sheet material conveying member onto the succeeding sheet material. To avoid this, in the conventional image forming apparatus of ink jet type, at a position where there is possibility of contacting with non-dried ink, there has been adopted an arrangement in which a roller driven by a driving source is arranged at an image non-formed surface side and a spur roller as a discharging rotary member provided at its periphery with a plurality of protruded portions is arranged at an image formed surface side, thereby pinching and conveying the sheet material.
However, in the image forming apparatus of full line type having the above-mentioned construction, when the sheet material having one surface on which the image was formed is reversely rotated and an image is also formed on the other surface, since the recording speed is high, so-called switch-back in which the conveying direction is reversed after the conveyance of the sheet material having one surface on which the image was formed is once stopped is effected, an adequate conveying force only by a roller and a spur roller, and, thus, when both-surface recording tries to be effected, sheet jam may occur at a sheet material reverse-rotating and conveying portion.
In this way, although there are sheet material conveying members optimum to surface conditions of the sheet material, in the conventional image forming apparatuses, the sheet materials having different surface conditions were conveyed by the same sheet material conveying member.